This invention relates to orthopedic surgery and more particularly to orthopedic fusion plates designed for rigid and semirigid fixation to encourage bony union when bone grafts are used in spinal fusion techniques.
Of all animals possessing a backbone, human beings are the only creatures who remain upright for significant periods of time. From an evolutionary standpoint, this erect posture has conferred a number of strategic benefits, not the least of which is freeing the upper limbs for purposes other than locomotion. From an anthropologic standpoint, it is also evident that this unique evolutionary adaptation is a relatively recent change, and as such has not benefitted from natural selection as much as have the horizontal backbones of other animals. As a result, stresses acting upon the human backbone (or vertebral column) are unique in many senses, and result in a variety of problems or disease states that are peculiar to the human species.
The human vertebral column is essentially a tower of bones held upright by fibrous bands called ligaments and contractile elements called muscles. There are seven bones in the neck or cervical region, twelve in the chest or thoracic region, and five bones in the low back or lumbar region. There are also five bones in the pelvic or sacral region which are normally fused together and form the back part of the pelvis. This column of bones is critical for protecting the delicate spinal cord and nerves, and for providing structural support for the entire body.
Between the vertebral bones themselves exist soft tissue structures-discs—composed of fibrous tissue and cartilage which are compressible and act as shock absorbers for sudden downward forces on the upright column. The discs allow the bones to move independent of each other, as well. The repetitive forces which act on intervertebral discs during repetitive day-to-day activities of bending, lifting and twisting cause them to break down or degenerate over time.
Presumably because of humans upright posture, their intervertebral discs have a high propensity to degenerate. Overt trauma, or covert trauma occurring in the course of repetitive activities disproportionately affect the more highly mobile areas of the spine. Disruption of a discs internal architecture leads to bulging, herniation or protrusion of pieces of the disc and eventual disc space collapse. Resulting mechanical and even chemical irritation of surrounding neural elements (spinal cord and nerves) cause pain, attended by varying degrees of disability. In addition, loss of disc space height reduces tension on the longitudinal spine ligaments, thereby contributing to spinal instabilities such as spinal curvature and lithesis.
The time honored method of addressing neural irritation and instability resulting from severe disc damage have largely focused on removal of the damaged disc and fusing the adjacent vertebral elements together. Removal of the disc relieves the mechanical and chemical irritation of neural elements, while osseous union (bone knitting) solves the problem of instability.
In the cervical spine, the most common type of fusion utilizes either bone dowels (Cloward Technique) or bone blocks (Smith Robinson Technique). These procedures have been used now for over four decades. One of the main causes of failure of these fusion techniques is the failure to fuse, or non-union, at the site where the bone is grafted between the vertebral bodies. In an attempt to circumvent this problem, various plate-type mechanism have been used both to provide instability, and to reduce or eliminate movement at the site of the fusion to allow successful bone knitting. These plating mechanisms function much as a cast on a fractured limb might provide support until healing can occur.
It is recognized that for bone knitting to occur, the interfaces of bone required to knit or heal must be held in close apposition and motion between the knitting or fusion interfaces must be restricted sufficiently for a certain minimal time period to permit stable bone union occur.
To achieve these ends, prior inventors have developed a variety of both external braces and internal fixation instruments, some in the form of plates. Internal fixation is advantageous in that it obviates the need for cumbersome external braces, collars or supports and ensures essentially total compliance. U.S. Pat. Nos. 5,041,113, 5,234,431, 5,344,421 and 5,681,311 provide examples of prior vertebral bone plate systems. U.S. Pat. No. 3,604,414 discloses a plate for setting fractures having separate elements which are attached to respective bone fragments and have a toothed interface to maintain the position of the fragments after they have been drawn together.
While interface apposition and retardation of motion are known to enhance bone healing, it is also recognized that if the bony surfaces to be fused are held together under a compression force, osseous fusion is further enhanced.
Even though many plating systems place bone grafts under sufficient compression to facilitate or even enhance bony fusion, bone grafts themselves may undergo minor degrees of resorption which causes the graft length to shorten or subside. This subsidence in turn is detrimental to the fusion process because the osteoblasts must now bridge an ever widening gap between host bone and graft. Ironically, rigid plating systems may actually work against the fusion process by holding host bone in a fixed position as subsidence occurs. To counteract this, many plating systems have resorted to semi-rigid systems that allow for screw and plate movement to counteract the effect of normal graft subsidence. That is, as the graft subsides, the screws above and below the graft are permitted to move via the force of gravity to allow continued bone graft apposition.
The trouble with most of these mechanisms is that they allow for too many degrees of movement in too many places. Whereas the only movement necessary to counteract subsidence is movement in a unidimensional plane, many existing plating mechanisms allow for movement of the screws in several planes, amounting in essence to putting in loose screws, which defeats the purpose of a true stabilizing plating system.
It would be desirable to have a plating mechanism which would provide the rigidity necessary to stabilize vertebral elements in appropriate alignment for fusion while simultaneously allowing for subsidence solely along the plane it would be expected to occur. This would be a clear advantage over the majority of plating systems which permit movement of component parts in so many dimensions that their value in providing stability and their utility in enhancing fusion is questioned. Additionally, by employing features that counteract subsidence through both active and passive modalities, fusion can be further enhanced.